Transducer probes are used, for example, for diagnostic sonography or “ultrasonography.” Diagnostic ultrasonography is an ultrasound-based diagnostic imaging technique used to visualize subcutaneous body structures including tendons, muscles, joints, vessels and internal organs for possible pathology or lesions. Obstetric ultrasonography is commonly used during pregnancy and is widely recognized by the public.
In physics, the term “ultrasound” applies to acoustic energy (longitudinal mechanical compression waves) with a frequency above the audible range of human hearing. The audible range of sound is generally considered to be about 20 hertz-20 kilohertz. Therefore, ultrasound is commonly considered to be frequencies greater than 20 kilohertz.
Ultrasonography uses a probe containing one or more acoustic transducers to send pulses of sound into a material. Whenever a sound wave encounters a material with a different density (acoustical impedance) part of the sound wave is reflected back to the probe and is detected as an echo. The time it takes for the echo to travel back to the probe is measured and used to calculate the depth of the tissue interface causing the echo. As a result, when the differences between the acoustical impedances become larger, so do the echoes.
The frequencies used for medical imaging are generally in the range of 1 to 18 MHz. Higher frequencies have a correspondingly smaller wavelength and can be used to make sonograms with smaller details. However, the attenuation of the sound wave is increased at higher frequencies so, in order to have better penetration of deeper tissues, a lower frequency range (e.g. 3-5 MHz) is often used.
A basic ultrasound machine includes a transducer probe (also known as a “probe head”) and a “main frame” including a CPU, transducer pulse controls, display, storage, I/O, etc. The transducer probe, which sends and receives the sound waves, is a critical component of an ultrasound machine. The transducer probe generates and receives sound wave using the piezoelectric effect. More specifically, in the probe there are one or more quartz (piezoelectric) crystals. When an alternating electric current is applied to these crystals, they change shape rapidly creating ultrasonic waves. Conversely, when ultrasonic waves impinge upon the piezoelectric crystals they create alternating electric current. Therefore, the same crystals can be used to send and receive sound waves. The probe also has a sound absorbing substance to eliminate back reflections and an acoustic lens to help focus the emitted sound waves.
A prior art ultrasound machine 10 is illustrated in FIG. 1 and includes a probe (“probe head”) 12, a main frame 14 and a cable 16 connecting the probe 12 to the main frame 14. The probe 12 includes a piezoelectric transducer 18. The main frame 14 includes a “pulser” transmitter 20 and a receiver 22. The cables often include 100-200 insulated wires to support various channels of the ultrasonic probe and to carry power, ground and other signals. Each wire dedicated to a channel can carry both high voltage transmission signals (HV TX Bursts) from the transmitter 20 and low voltage receive signals (LV RX Bursts or “echoes”) received by the receiver 22. The HV TX Bursts are typically 200 volts peak-to-peak, while the LV RX Bursts are typically no more than ±300 millivolts. Signals from the various channels are processed by the mainframe to create the ultrasound image.
Prior art commercial probes, such as probe 12, are generally passive, i.e. amplification has traditionally only been performed in the main frame 14. Furthermore, the probe 12 is not typically impedance matched to the cable 16. Most probes have an impedance of 300-500Ω while cables have an impedance of about 75Ω. This results in a high amount of attenuation of the LV RX signals, a high signal-to-noise ratio (SNR) and signal reflection problems.
Amplification has not been used in commercial ultrasonic probes for a variety of reasons. For one, the high voltage bursts from the mainframe transmitter can be very damaging to delicate semiconductor devices such as operational amplifiers. For another, there are severe space and power restrictions in probes heads. That is, because the probes are handheld and are pressed against the skin, they cannot be made too large, nor can they generate too much heat. The size and heat generation of electronic circuitry is contrary to these requirements. Also, any probe with amplification would have to remain compatible with standard ultrasound machines, complicating design choices.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.